First, it is remarkable that the school is encouraging this for your son.
That is a good sign, in my estimation.

How about thinking of this as a trial period vs a momentous irreversible decision? Could you suggest that to the school? That may take some pressure off and allow clearer thinking.

Children change classes mid year all the time, sometimes for acceleration (a boy in my son's class was moved up a month or so into the school year), family moves or school changes.